The production of oil and natural gas from an underground well (subterranean formation) can be stimulated by a technique called hydraulic fracturing, in which a viscous fluid composition (fracturing fluid) containing a suspended proppant (e.g., sand, bauxite) is introduced into an oil or gas well via a conduit, such as tubing or casing, at a flow rate and a pressure which create, reopen and/or extend a fracture into the oil- or gas-containing formation. The proppant is carried into the fracture by the fluid composition and prevents closure of the formation after pressure is released. Leak-off of the fluid composition into the formation is limited by the fluid viscosity of the composition. Fluid viscosity also permits suspension of the proppant in the composition during the fracturing operation. Cross-linking agents, such as borates, titanates or zirconates are usually incorporated into the composition to control viscosity.
Normally, less than one third of available oil is extracted from a well after it has been fractured before production rates decrease to a point at which recovery becomes uneconomical. Enhanced recovery of oil from such subterranean formations frequently involves attempting to displace the remaining crude oil with a driving fluid, e.g., gas, water, brine, steam, polymer solution, foam, or micellar solution. Ideally, such techniques (commonly called flooding techniques) provide a bank of oil of substantial depth being driven into a producing well; however, in practice this is frequently not the case. Oil-bearing strata are usually heterogeneous, some parts of them being more permeable than others. As a consequence, channeling frequently occurs, so that the driving fluid flows preferentially through permeable zones depleted of oil (so-called “thief zones”) rather than through those parts of the strata which contain sufficient oil to make oil-recovery operations profitable.
Difficulties in oil recovery due to high permeability of zones may be corrected by injecting an aqueous solution of an organic polymer and a cross-linking agent into certain subterranean formations under conditions where the polymer will be cross-linked to produce a gel, thus reducing the permeability of such subterranean formations to driving fluid (gas, water, etc.). Polysaccharide- or partially hydrolyzed polyacrylamide-based fluids cross-linked with certain aluminum, titanium, zirconium and boron-based compounds are also used in these enhanced oil recovery applications.
Cross-linked fluids or gels, whether for fracturing a subterranean formation or for reducing permeability of a subterranean formation, are now being used in wells under a variety of temperature and pH conditions, where rates of cross-linking with known cross-linking compositions may be unacceptable.
U.S. Pat. No. 4,578,488 discloses adding water to a cooled solution of a zirconium complex having a mole ratio of triethanolamine:zirconium of 2:1 to 1:1, preferably, 2:1, with the amount of water being added at a mole ratio of water:zirconium of 3:1 to 0.5:1. In one example, in which water is added at 55° C., a hazy product resulted. The cross-linking rates of the triethanolamine zirconium complexes upon addition of water were very fast, generating highly viscous gels.
U.S. Pat. No. 4,683,068 discloses preparing a zirconium complex of triethanolamine under anhydrous conditions, at a mole ratio of triethanolamine:zirconium of 2-3:1. The zirconium complex is activated by addition of large volumes of water, in excess of 600-1000 moles water per mole of zirconium, to provide an active cross-linking agent which cross-links “almost instantaneous” when blended with a polysaccharide (hydroxypropylguar). This patent further discloses that the activated zirconium complexes have poor shelf life.
U.S. Pat. No. 4,686,052 discloses a process to stabilize zirconium cross-linking complexes in water by adding a large excess of an alkanolamine, such as triethanolamine. The mole ratio of triethanolamine:zirconium for stabilizing in the presence of water is at least 15:1, preferably at least 42:1, if cross-linked gel will be exposed to temperature>200° F. (93° C.). These complexes result in very slow cross-linking. The high loading of triethanolamine also renders these complexes undesirably expensive.
There is a need for compositions which cross-link at a desirable rate, especially within the range of 3-8 minutes, and that such cross-linking compositions are economical and stable in the presence of water. The present invention meets these needs.